This invention addresses the problem of film dust formation during the patching of polystyrene window envelope film. A window envelope is an envelope with one or more openings of any shape, usually rectangular, which allows examination of any information, typically a name and an address, printed on a limited area of matter within. The opening or openings are sealed or closed by a window patch composed of a non-opaque plastic film, usually polystyrene.
Patching is the process in which window envelope film is cut into patches of a desired length and width, gummed around the edges, and applied to the window envelope. Alternatively, the edges of the window opening or openings may be gummed instead of the edges of the window patch. The patching process requires the use of a vacuum drum to ensure proper positioning of the film. Film dust results from abrasion and scratching of the passing film by the surface of the vacuum drum due to the disparity in the speed of the passing film and the angular velocity of the drum. The abrasions and imperfections on the surface of the vacuum drum fleck away particles from the surface of the passing film resulting in film dust.
Film dusting was not a serious problem until recent advances in the operating speed of window envelope film patching units. Older units operated at about 400 envelopes per minute while modern units typically operate at about 800-1000 envelopes per minute. The increased scratching and abrasion of the passing film by the vacuum drum exacerbated the dusting phenomena to the point of being problematic.
The vacuum drum on a typical commercial patcher has a multiplity of minute vacuum holes and grooves on its surface to effect the adherence thereto and guidance thereon of the passing film or film patches. Vacuum holes are recessed in the grooves, which traverse the surface of the vacuum drum in both the machine and cross-machine directions relative to the passing window envelope film. The film dust accumulates in the vacuum holes and grooves and on the raised surfaces of the drum frustrating the adhering effects of the vacuum. Film dust also accumulates at a lesser rate in the internal areas of the vacuum drum and in the apparatus creating the vacuum. Process downtime is required to clean the vacuum drum and associated vacuum implementation equipment.
Window envelope films are typically composed primarily of extruded polystyrene, and, optionally, with a small proportion of a rubber-reinforced polymer to add a cosmetically desirable haze or measure of translucency to the film with the measure of translucency being proportional to the concentration. Window envelope film may be manufactured in both biaxial and uniaxial orientations. Biaxial orientation is generally preferred because of the superior cuttability of the film in the transverse or crossmachine direction.